I have an idea on how his rib cage is positioned, but not too sure on his pelvic bone. It looks like the front of his pelvis is actually facing to the right and perhaps a tiny bit towards the back (red background) and a bit to the ground. It looks like there is a bit of foreshortening in the waist area, but not too sure. This is a very hard pose for me and the clothing doesn't help at all.
I would love to upload a sketch, but it's nothing like the picture, completely off.
Hello! This pose is definitely a challenge, but do-able with the right approach. I would begin by outlining the torso in what is known as 'the peanut' technique, (or some other very silly title.) In the technique, you draw two orb shapes, one for the rib cage, and one for the hips. You add a contour center line and connect the two in what usually looks like a peanut. What I know about it I learned from Matt A. on Drawing Tutorials Online (Video link below)
Here's what I came up with. The shape is still a bit difficult for me to understand and convey simply, as in a gesture, but I was able to identify a center line which helped out a lot!
I think you are overthinking the task. A gesture is just for an action line and briefly putting down other points. If you want to sketch, that is different. I was always taught that lines of action are either a straight line, a C line or an S, following the spine and one or two limbs. This major line will show the action of the pose and the resulting drawing will be different depending on the line you choose or the line you see. In the referenced photo I see a strong C line starting with the spine and continuing with the right leg going forward. You could also use a tighter C line you could start with the right arm, through the spine and back through the left leg, but to me this pushes the action backwards. Once you have the action line then you can add the limbs, head, torso and hips shapes. This is where you will get the twist in the torso. You should try just getting the action lines down. Then try it using different a line. Even maybe an S curve starting with the head. You will see that the action line will drive your sketch. You can even change the position of a limb to get a different look (i.e. reposition the left arm as if it were extending forward parallel with the right leg) then you could have a full S curve going from the left arm through the spine and back through the left leg. Once you get to where you can immediately see the lines of action, you can then start to use those lines to "gesture" draw from your imagination and create your own poses.